A. Field of Invention
The present invention is related to voice over IP communication systems, and more particularly, to a method and system of providing IP-based enhanced emergency services using intelligent client devices.
B. Description of Related Art
Enhanced emergency services for telephony systems tie the caller's physical location to the call signaling and follow-on messaging. The goal is to increase the effectiveness of the emergency response personnel. By helping pin-point the caller's location, as well as adding reliability to the telephony link between the caller and the emergency responder, precious time may be saved in responding to an emergency. In the United States, the service is referred to as E-911.
While government and industry groups have worked together to provide consistent requirements of the E-911 system, many aspects of practical implementations have not yet achieved standardization. The variety of systems currently deployed may share common elements in their respective designs, but optimal solutions are still lacking for many of the technical challenges posed by the requirements. This is particularly true for an IP telephony system in an enterprise or campus environment.
The successful delivery of E-911 service requires that two general areas of operation be satisfied: 1) the ability to route 911 calls to an appropriate emergency response center, based upon the location of the caller (calling station); and 2) the ability of the emergency response center to both locate, and automatically call back to, the calling station, following the receipt of a 911 call from that station (call-back being required, e.g., in the event that the original call gets disconnected). Both of these are related to each other by virtue of their dependency on the location of the calling station. Therefore, accurate and verifiable location of the caller is fundamental to proper implementation of E-911 service.
1. Emergency E911 Terminology
The definitions below reproduce E-911 terminology listed at the Association of Public-Safety Communication Officials (APCO) website. The list may be accessed at the following Internet website at www.apco911.org/about/pbx/index.html:    9-1-1: A three digit telephone number to facilitate the reporting of an emergency requiring response by a public safety agency.    9-1-1 Service Area: The geographic area that has been granted authority by a state or local governmental body to provide 9-1-1 service.    9-1-1 Service Provider: An entity providing one or more of the following 9-1-1 elements: network, CPE, or database service.    9-1-1 Tandem: (See E9-1-1 Control Office)    Access Line The connection between a customer premises network interface and the Local Exchange Carrier that provides access to the Public Switched Telephone Network (PSTN).    Automatic Location Identification (ALI): The automatic display at the PSAP of the caller's telephone number, the address/location of the telephone and supplementary emergency services information.    Automatic Location Identification (ALI) Database: The set of ALI records residing on a computer system.    Automatic Number Identification (ANI): Telephone number associated with the access line from which a call originates.    Central Office (CO): The Local Exchange Carrier facility where access lines are connected to switching equipment for connection to the Public Switched Telephone Network.    Centralized Automated Message Accounting (CAMA): An MF signaling protocol originally designed for billing purposes, capable of transmitting a single telephone number.    Data Base Management System Provider: Entity providing Selective Routing (SR) and/or Automatic Location Identification (ALI) data services.    Data Provider An entity which provides, on a routinely maintained static database, names, addresses and telephone number to be inserted and updated in the E911 MSAG. Data providers are defined as local exchange carriers, alternate exchange carriers, wireless carriers or an entity authorized to act on behalf of any of the aforementioned entities.    Default Routing The capability to route a 9-1-1 call to a designated (default) PSAP when the incoming 9-1-1 call cannot be selectively routed due to an ANI failure or other cause.    Enhanced 9-1-1 (E9-1-1) Control Office: The Central Office that provides the tandem switching of 9-1-1 calls. It controls delivery of the voice call with ANI to the PSAP and provides Selective Routing, Speed Calling, Selective Transfer, Fixed Transfer, and certain maintenance functions for each PSAP. Also known as 9-1-1 Selective Routing Tandem or Selective Router.    Emergency Location Identification Number (ELIN): A valid North American Numbering Plan format telephone number assigned to the MLTS Operator by the appropriate authority that is used to route the call to a PSAP and is used to retrieve the ALI for the PSAP. The ELIN may be the same number as the ANI. The North American Numbering Plan number may in some cases not be a dialable number.    Emergency Response Location (ERL): A location to which a 9-1-1 emergency response team may be dispatched. The location should be specific enough to provide a reasonable opportunity for the emergency response team to quickly locate a caller anywhere within it.    Emergency Service Number (ESN): A number assigned to specific geographic area within which all E911 calls are routed to one specific PSAP and the residents of the area are served by the same police, fire, and emergency medical agencies.    Emergency Service Zone (ESZ): The geographic area within which all E911 calls are routed to one specific PSAP and the residents of the area are served by the same police, fire, and emergency medical agencies.    Fast Busy: (see Reorder Tone)    Grade of Service: The probability (P), expressed as a decimal fraction, of a telephone call being blocked. P.01 is the grade of service reflecting the probability that one call out of one hundred during the average busy hour will be blocked. P.01 is the minimum recommended Grade of Service for 9-1-1 trunk groups.    Master Street Address Guide (MSAG): A data base of street names and house number ranges within their associated communities defining Emergency Service Zones (ESZs) and their associated Emergency Service Numbers (ESNs) to enable proper routing of 9-1-1 calls.    No Record Found (NRF): A condition where no ALI information is available for display at the PSAP.    P.01 Grade of Service: (See Grade of Service.)    PBX: (See Private Switch)    Primary Public Safety Answering Point (PSAP): A PSAP to which 9-1-1 calls are routed directly from the 9-1-1 Control Office. (See PSAP)    Primary Rate Interface (PRI): Primary Rate Interface (PRI) is trunking technology which enables the networking of multiple locations. A single PRI trunk can carry various types of traffic. PRI provides such features as Calling Number Delivery, Called Number delivery, Network Redirection and Reason, Network Name, Network Ring Again, Network Automatic Call Distribution, Equal Access, Special number Services, Integrated Service Access (ISA), Network Message service, and Release Link Trunk (RLT). Each PRI trunk group requires one D-Channel and can support multiple DS-1s up to a maximum of 479 B-channels distributed over 20 DS-1 links. PRI call processing supports Q.931 messages for call setup, call progress, and feature activation.    Private Switch ALI (PS/ALI): A service option which provides Enhanced 9-1-1 features for telephone stations behind private switches. e.g. PBXs    Public Safety Answering Point (PSAP): A facility equipped and staffed to receive 9-1-1 calls. A Primary PSAP receives the calls directly. If the call is relayed or transferred, the next receiving PSAP is designated a Secondary PSAP.    Public Switched Telephone Network (PSTN): The network of equipment, lines, and controls assembled to establish communication paths between calling and called parties in North America.    Reorder Tone An audible tone of 120 interrupts per minute (ipm) returned to the calling party to indicate the call cannot be processed through the network. Sometimes referred to as fast busy.    SCC: The Qwest 9-1-1 database management service provider.    Selective Routing (SR): The routing of a 9-1-1 call to the proper PSAP based upon the location of the caller. Selective routing is controlled by the ESN which is derived from the customer location.    System Integrator Coordination and oversight responsibilities as undertaken by the Company relating to the quality of 911 serviced provided by the Company, alternate exchange carriers and data providers.
2. E-911 Service Requirements
The primary elements of E-911 service are: 1) association of locations with 911 calls; and 2) routing 911 calls to emergency response centers most suited to answering and responding (including dispatching emergency personnel) to specific calls. The emergency response center is referred to as the Public Safety Answering Point, or PSAP. The mechanisms of E-911 service include transport of vital and relevant information in the call signaling, using this information to route the call to the optimal PSAP, and presenting this information to the PSAP personnel to help determine the location of the caller.
When a 911 call is placed, the location of the caller is used to route the call to an appropriate end-office switch. The calling station location is referred to as the Emergency Response Location, or ERL. The routing can be static, e.g., as in the case of a residential connection to a specific end-office; or dynamic, e.g., as in the case of a lookup by a PBX system based upon the calling station extension. Included in the call signaling is a phone number called the Emergency Location Identification Number, or ELIN, that identifies the ERL from which the call was placed. The ELIN is included in the call signaling of a 911 call as the ANI. In the PSTN, the ELIN is used to route the call to an appropriate PSAP. The routing is done by accessing a location information database called the called Automatic Location Identification (ALI) Database. The ELIN may also be used by the responding PSAP to access the ALI for detailed location information. The PSAP may also use the ELIN to call back the calling station in the event that the call gets disconnected.
In addition to the procedures used to set up a 911 call and determine the location of the caller, the system must insure that emergency calls cannot be disrupted by signaling events which might be allowable for non-emergency calls. For example, if the caller has call-waiting service, it must be disabled during an emergency call from that caller's phone. Similarly, only the PSAP, or its representative, may release a 911 call; if the caller hangs up the phone during a 911 call, the call should not be released. Other potentially interrupting service features must similarly be disabled. The PSTN typically provides these capabilities.
Typically in a PBX system, an on-site emergency facility will be notified when a 911 call is placed. This may be a simple logging system, or a security system that is monitored by security/safety personnel. The information available to such a system may include more detail than that passed to the PSTN, or maintained in the ALI database. For example, the location information available to the on-site system may be precise enough to identify an office or cubicle, while the ERL available to the PSAP may only identify a building floor or wing. Reliance on an on-site component to a PBX emergency response system is one approach to mitigating some of the difficulties in designing such a system.
Deployment of E-911 behind a PBX introduces further considerations such as location precision, mapping ELIN to calling station, and the information in the ALI database. As noted, the specification of the location of a 911 calling station is defined as the Emergency Response Location, or ERL. The precision with which the ERL actually locates a calling station may vary. For example, for residential service, an ERL may be the address of a house, or a unit within an apartment complex. For an enterprise PBX, an ERL could be a building address, a floor in building, a wing on a floor, or even an office or cubical.
When an ERL corresponds to an extended area, such as building floor or wing, it may contain multiple calling stations. Note that the information used both to route a 911 call to a PSAP, as well as to provide the PSAP with the location of the calling station, is only as precise as the ERL from which the call is placed. Considerations in specifying an ERL can include state and local regulations governing required precision of the location, and, in the case of multi-station ERLs, the maximum number of calling stations that a single ERL may contain. Recall that the identity of an ERL is associated with the ELIN, which acts as a phone number. Note that one ERL may possess more than one ELIN, but one ELIN may identify one and only one ERL.
Within the PSTN, location information is maintained in a database called the Automatic Location Identification (ALI) Database. Typically, there are multiple ALI databases, each associated with, e.g., a different calling area, or different local exchange carrier (LEC). The ALI database essentially correlates ELINs with ERLs, as well as with routing rules for directing 911 calls to appropriate PSAPs. Thus the ALI database is queried using the ELIN when a 911 call is made to determine which one of possibly multiple PSAPs should receive the call. Once the call is received by the PSAP, the ELIN may again be used to query the ALI, this time to provide ERL information. The ELIN can also be used by the PSAP as a direct call back number to the calling station, e.g., in the event that the original call is disconnected. That is, as far as the PSAP is concerned, the ELIN functions as a Direct Inward Dial (DID) number to the calling station. Note that in the case of multi-station ERLs, a single ELIN may be shared among all calling stations in an ERL. Therefore, in order for the PSAP to be able to use the ELIN as a DID number to refer to a specific calling station, some form of mapping from ELIN to calling station must be implemented by the PBX.
When a 911 call is placed from behind a PBX, the ERL of the calling station is the factor that determines the ELIN that will be passed to the end-office switch; i.e., into the PSTN signaling space. Before the call is actually passed to the PSTN, the PBX, or some associated on-site emergency service system, must first select an appropriate end-office switch. If there is only one choice (i.e., the PBX is connected to only one switch), then the decision is pre-determined. If there is more than one, then the on-site system must be configured to route 911 calls to end-office switches based upon ELIN (or ERL). When the switch receives the call, it (or an agent of the switch) consults the ALI database to determine the proper PSAP to which the call should be routed.
Once the PSAP receives the call, the location and callback information it gets is only as specific as is contained in the ELIN and the ALI database. That is, any site-specific mapping or location information that is not contained in the ERL definition in the ALI database is not passed in the call. Typically, calls outbound from a PBX do not pass internal extension information. This means that if an ERL contains several calling stations, the ALI will only locate the ERL, not individual calling stations. Also, even if each calling station has its own DID number, the PSAP will only receive the ELIN of the ERL; the on-site emergency system must incorporate intelligence to recognize that a 911 call has been made, and, if multi-station ERLs are used, to establish and maintain a mapping of ELIN to DID of the calling station (to support the event of call disconnection to the PSAP). Depending upon the level to which internal location information is passed in outbound 911 calls, a PBX may actually have to multiplex call-back numbers across internal location areas, rather than provide location-unique information to emergency response centers. To complicate matters further, the allowable size and definition of such internal areas may be subject to different regulatory rules in different states or localities.
Unless each calling station is its own ERL, then sharing of ELINs among calling stations leads to the possibility of more than one concurrent 911 call using a single ELIN. While the ERL information in such a case would be correct, the callback information would be ambiguous, since the PSAP would have no way of uniquely identifying the calling station beyond ELIN, and thus the PBX would not be able to de-multiplex a callback from the PSAP based upon ELIN alone. Note that while the ALI database must be regularly updated to reflect configuration changes in the PBX system, such updates are not intended to provide dynamic mapping on a call-by-call basis.
One way to deal with the case of multiple calls per ERL is to assign multiple ELINs to multi-station ERLs. Such a technique is used in the Emergency Response E-911 system by Cisco, for example. The number of ELINs per ERL can be determined according to the statistical probability of concurrent 911 calls in an ERL of a given size. As long as the number of ELINs assigned to an ERL is always greater than or equal to the number of concurrent 911 calls from that ERL, then there can always be a unique mapping of ELIN to calling station. One limitation is that “always” can not be guaranteed except for the case of single-station ERLs. Given multi-station ERLs, the issue of the number of ELINs provisioned for each ERL is primarily one of cost: the PSTN service provider charges for each ELIN assigned. Thus for multi-station ERLs, there is a tradeoff between a “safe” ratio of calling stations to ELINs, and the cost of ELINs. The importance or relevance of this tradeoff is partially dependent upon the size and type of PBX.
For example, in a traditional (TDM) PBX system that provides each calling station with a DID number, single-station ERLs are not necessarily an excessive allocation of circuit resources. Since the service provider charges for each DID number, the PBX can, in principle, use each DID as an individual ELIN, thereby providing each calling station with its own ERL (and ELIN). This can work because, in a circuit-based PBX, even though each DID is typically associated with a person (e.g., employees at an enterprise), each is also assigned to a specific calling station. Because a 911 call is tied to the ERL of the calling station, the DID number identifies the ERL.
The situation is different in an IP-based PBX that uses, e.g., SIP. In this case, even if each person served by the PBX has an individual DID number, that number is not necessarily tied to a specific calling station (i.e., SIP phone). Rather, each DID number is associated with a person, and any person could register at any SIP phone behind the PBX. As a result, no predictable association exists between personal DID numbers and ERLs. It is still possible to tie each calling station to an ERL by virtue of some hardware attribute and assign an ELIN to the ERL. However, in order to support single-station ERLs in this environment, a complete second set of hardware-identifying DID numbers would be required to assign one each to every calling station. While this would free personal DIDs to travel with each user, it could be expensive, and requires a burdensome level of provisioning of circuit resources.
In addition to the problems associated with the ELIN-DID mapping in multi-station ERLs in IP-based PBXs, LAN system and equipment configurations tend to be more fluid, in general, than circuit-based PBX systems. Layer ⅔ switches can be moved or reconfigured, IP subnets can be modified, and SIP phones can be relocated. Within the context of E-911 service, this characteristic has the potential of translating into problems or difficulties in maintaining accurate location information, even for calling stations. Some solutions, such as the Cisco Emergency Responder, utilize a centralized device (e.g., the Emergency Responder node) to query a network device (e.g., the CallManager) to obtain a list of registered phones. The central device then queries individual switches to determine which physical ports are associated with those phones. Because a user's location may change at any moment, the central device must repeat this process continuously. If the time between queries is short, the amount of network traffic can become large, and if the time interval is too long, the risk of improperly reporting the ERL increases.
A LAN-based or VoIP-based PBX can introduce additional problems because internal extension numbers, which are usually associated with a person, may not be accurate indicators of location. For example, in the case of a SIP-based system, a given end user with a “fixed” phone number may actually register at arbitrary locations within network. If such a user makes a 911 call, then the fixed phone number associated with that user may provide inaccurate, or worse, dangerously erroneous, location information. To address this problem, a 911 caller's phone number must be mapped to a physical attribute of the phone, such as MAC address, which in turn may be associated with a current network location.
While conceptually straightforward, this solution brings with it added complexities, such as a new layer of indirection in the mapping, possible delay issues, and the need for a method for reliable location tracking of “portable” phones, among others.
In addition, reliance on network-based call control elements in setup of 911 calls can introduce a failure point. While redundant elements may be added, this increases the expense and configuration of the E911 system. It also requires call control elements to reside on the same site as the phones in order to avoid WAN-based communications to complete a 911 call. In turn, this may add further expense for small sites that could otherwise use WAN-based communications for non-emergency calls. Consequently, a system that overcomes these limitations is desirable.